*{
  box-sizing: border-box;
  padding: 0;
  margin: 0;
}
.nav{
  /* 去除两个button间的缝隙 */
  font-size: 0;
}
.nav button{
  width: 50%;
  height: 40px;
}
.mian{
  
}
.center{
  display: none;
}